Conventional diagnostic use of x-radiation includes the form of radiography, in which a still shadow image of the patient is produced on x-ray film, and fluoroscopy, in which a visible real time shadow light image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient.
In a typical x-ray tube, electrons are generated from a filament coil heated to thermionic emission. The electrons are accelerated as a beam from a tube cathode through an evacuated chamber defined by a glass envelope, toward a target anode. When the electrons strike the anode with large kinetic energies and experience a sudden deceleration, x-radiation is produced. An x-ray tube assembly is contained in a housing which includes a window transmissive to x-rays, such that radiation from the anode passes through the window toward a subject undergoing examination or treatment.
Most x-ray tube designs employ filaments as a source of electrons. A filament is a coil of wire which is electrically energized so that electrons are thermionically emitted from the filament and accelerated toward the anode due to a very large DC electrical potential difference between the cathode and the anode. Often this electrical potential difference is of the order of 150,000 volts, (.+-.75,000 volts to ground) necessitating significant electrical isolation between the various tube components.
Despite the electrical isolation, when two elements with 150 kV difference in potential are placed proximate to each other, there is a tendency to arc. An arc is an undesired surge of electrical current between two elements which are at a different electrical potential and typically occurs through gas molecules present in the x-ray tube. In x-ray tubes, this tendency to arc often increases as the tube ages due to such factors as degradation of the vacuum within the tube due to the existence of additional undesired gas molecules. When the x-ray tube arcs, a current on the order of hundreds of amperes can flow between the cathode and the anode. Once an x-ray tube starts to arc, an avalanche type effect may occur sputtering metal and the metal atoms as well as ionizing the contaminants in the vacuum.
Arcing typically occurs in the area of the x-ray tube having the highest electric field strength. As such, arcing in an x-ray tube will commonly occur in the same region as where the cathode is supplying the anode with electrons for the production of x-ray emissions. The sputtering of metal from the cathode produced during arcing often lands on the internal surface of the glass envelope in proximity to the cathode. The existence of the metal deposits on the glass envelope can deleteriously effect x-ray tube performance for several reasons. First, as arcing occurs from time to time, sputtered metal deposits will continue to grow. As the sputtered metal deposits on the glass envelope gets too thick, an electrical charge may accumulate sufficient to damage the glass envelope thereby rendering the tube non-functional. Secondly, sputtered metal deposits on the glass envelope will often attract arcing between the deposits and the cathode. The surges of electrical current produced during arcing can damage the glass envelope, again rendering the tube non-functional. Additionally, the metal deposits also cause the glass surface area below the metal deposits to remain at a higher temperature than the glass would normally remain without such metal deposits. The higher glass temperatures in these regions causes further instability in the glass.
Attempts to reduce these and other negative effects of sputtered metal on the glass envelope include a method described in U.S. Pat. No. 4,315,182 ('182) assigned to Picker International. In the '182 Patent a method of roughing the internal surface of the glass envelope is described which causes the sputtered metal deposits to be spread out thereby helping to prevent charge build up on the glass envelope. Although the method described in the '182 Patent does offer significant benefits to maintaining the integrity of the x-ray tube, further improvements are still possible.
Attempts have also been made to reduce arcing. One known method to reduce arcing involves providing getter material inside the glass envelope to help maintain the evacuated state. The getter material binds gases on its surface and absorbs such gases to maintain the vacuum state in the x-ray tube. The process of removing residual gases from an evacuated area by binding and absorbing is known as pumping. By using getter material to maintain a vacuum state, arcing is minimized since there is a reduction in the number of gas molecules through which large current surges may flow. Unfortunately, as the x-ray tube ages the effectiveness of the getter material in pumping also diminishes. As a result, arcing tends to become more frequent as the tube ages.
Therefore, what is needed is a method and apparatus for maintaining the integrity and reliability of the x-ray tube as it ages. More specifically, what is needed is a method and apparatus for minimizing the amount of sputtered metal which accumulates on the envelope of an x-ray tube and for increasing the gas pumping ability of an x-ray tube which reduces and/or resolves the above-referenced difficulties and others.